1. Field of the Invention
The present invention relates to a liquid droplet measurement apparatus and a liquid droplet measurement method, and more particularly to a liquid droplet measurement apparatus and a liquid droplet measurement method of measuring the volume, velocity and position of a liquid droplet ejected from a print head of a recording apparatus such as an inkjet printer.
2. Description of the Related Art
In the related art, the following inventions have been disclosed as methods and apparatuses for measuring the volume, velocity or position of liquid droplets ejected from a print head.
Japanese Patent Application Publication No. 2006-281714 discloses a method and an apparatus whereby laser light is irradiated onto liquid droplets, the laser light which has passed through the liquid droplets is input to a determination device via a slit, and the volume, velocity or position of the liquid droplets is measured on the basis of the change in the output signal from the determination device.
Japanese Patent Application Publication No. 2005-83769 discloses a method and an apparatus for measuring the volume, velocity or position of liquid droplets by irradiating a plurality of laser light beams in a direction perpendicular to the direction of ejection of the liquid droplets, ejecting liquid droplets between these plurality of laser light beams, and ejecting liquid droplets so as to traverse the plurality of laser light beams.
More specifically, liquid droplets are ejected between a pair of laser light beams, and if a liquid droplet is determined by either of the beams, then it is judged that the liquid droplet has been ejected on a deviated path. Japanese Patent Application Publication No. 2005-83769 also discloses that the amount of deviation and the volume (outer shape) of the liquid droplet are determined by gradually altering the interval between the laser light beams, a small amount at a time. Furthermore, Japanese Patent Application Publication No. 2005-83769 also discloses that a method of judging whether or not there is a large amount of deviation on the basis of whether or not there is a large change in the amount of laser light received, in an example where the interval between the laser light beams is fixed.
However, the intensity of light is not uniform throughout the cross-section of a laser light beam, and there is a light intensity distribution in which the light intensity declines from the central portion of the cross-section toward the outer sides.
Therefore, in Japanese Patent Application Publication No. 2006-281714 and Japanese Patent Application Publication No. 2005-83769, if there is a marked light intensity distribution of this kind, then variation occurs in the measurement results produced by the determination device and it is not possible to measure the characteristics, such as the volume, velocity or position of the liquid droplets, accurately. Furthermore, in Japanese Patent Application Publication No. 2005-83769, it is necessary to adjust the position of the light beam finely in order to measure the volume, and this requires time and labor.
Japanese Patent Application Publication No. 2005-83769 discloses judging that there is a large amount of deviation when there is a large change in the amount of laser light in a case where the interval between the laser light beams is fixed, but it does not give a detailed description of the judgment method used.
Furthermore, in Japanese Patent Application Publication No. 2006-281714, a composition is adopted in which a signal which has passed through a liquid droplet is obtained by placing a slit in the light path, and therefore it is necessary to ensure that the intensity distribution of the light entering into the slit is uniform. If there is an intensity distribution, then this produces error in the determination of the transit timing of the droplet passing the slit. Furthermore, in an inkjet head, in order to measure liquid droplets (and in particular, in order to measure the transit position of the liquid droplet), it is necessary to perform the measurement at the distance where the paper is actually to be positioned, and in general this is 0.5 to 2 mm. However, Japanese Patent Application Publication No. 2006-281714 does not disclose a method of achieving a uniform light intensity distribution at a close position of this kind. Moreover, since the shielding of the light by the slit causes a reduction in the amount (intensity) of light incident on the sensor and thus causes the sensor output to fall, then it is difficult to improve the S/N ratio of the signal. On the other hand, if the width of the slit is enlarged, then the amount of incident light increases and the SIN ratio improves, but in the method described in Japanese Patent Application Publication No. 2006-281714, there is a fall in the accuracy of determining the transit of the liquid droplet.